Zishen Yutai Pill for Patients With Recurrent Implantation Failure

The goal of this clinical trial is to evaluate the efficacy of Zishen Yutai Pill (ZYP) on pregnancy outcomes following embryo transfer and its safety in patients with recurrent implantation failure. The main question it aims to answer is whether ZYP can improve live birth rate in participant's frozen embryo transfer (FET) cycles. Researchers will compare ZYP to a placebo (a look-alike substance with similar characteristics to ZYP) to see if it works to improve pregnancy outcomes. Participants will: 1. Start to receive ZYP or placebo (5g per time, 3 times daily) within the first 5 days of their initial menstrual cycle, and will undergo FET in the following cycle. The medication will be taken without interruption till the day of pregnancy test (2 weeks after embryo transfer). Patients with positive results in β-HCG test will continue to take the drug until clinical pregnancy confirmation by ultrasound three weeks later. For those with a negative β-HCG result, the intervention will be stopped. 2. Baseline visit will be conducted on days 2-4 of the participant's first menstrual cycle. Subsequent clinic visits will follow the standard protocol for FET cycle, as outlined below: Visit 1 (days 2-4 of the second menstrual cycle); Visit 2 (day of ovulation or day of endometrial transformation); Visit 3 (day of embryo transfer); Visit 4 (2 weeks after embryo transfer); Visit 5 (5 weeks after embryo transfer). Follow-up is scheduled at 10 weeks after embryo transfer (Visit 6) and after delivery (Visit 7), and these can be conducted remotely.